In recent years, the performance of an electronic appliance such as a personal computer, a mobile phone, and PDA has been remarkably improved, and this improvement depends on remarkably improved performance of a CPU. With such improved performance of the CPU, the amount of heat generated by the CPU has also been remarkably increased. Therefore, an important problem is how to dissipate heat in the electronic appliance.
As countermeasures of heat, there are methods such as air cooling by a fan, a heat pipe, and water cooling using water. However, the methods all require a new device for heat dissipation, and as a result, have defects in which the weight of the appliance may be increased and additionally, noise, the quantity of used electricity, and the like may be increased.
On the other hand, a method for diffusing heat generated by the CPU into a wide surface as rapidly as possible is to increase the cooling efficiency, and is the most realistic method as a cooling method in the electronic appliance such as a mobile phone and a personal computer.
In recent years, in a display device typified by an organic EL element, the size has been increased, and the precision of the device, especially uniformity, has been considered important. In particular, in the organic EL element, since the element itself is constituted by an organic substance, degradation due to heat has been known to affect the life of the element, and especially light-emitting properties and a change of chromaticity. Heat generated by a drive circuit constituting the device, or the like, is accompanied by a temporal change.
As a thermal conduction sheet used for such heat dissipation, much attention has been recently directed to sheet-shaped graphite.
This is because a good-quality graphite sheet has a very high thermal conductivity of 100 to 1,000 W/(m·K) and remarkably high performance as compared with properties of thermal conductivity of gelled heat dissipation material and sheet-shaped heat dissipation material other than the graphite sheet, and is the most suitable to diffuse heat.
Patent Literature 1 describes a thermal conduction sheet for directly transmitting heat from a heating element to a heat dissipation member. Patent Literatures 2 and 3 describe a thermal conduction sheet having heat diffusion properties of diffusing heat from a heating element in a plane direction.
As a thermal conduction sheet, attention has been paid to the thermal conduction sheets described in Patent Literatures 2 and 3, not in Patent Literature 1, and in particular, a so-called graphite sheet in which graphite is produced into a sheet.
Specifically, the graphite sheet has a high thermal conductivity of 100 to 1,000 W/(m·K) in a plane direction and allows heat from the heating element to be diffused to make the temperature within the electronic appliance uniform. Thus, the graphite sheet prevents a decrease in function of a component disposed in the appliance.
A third embodiment of Patent Literature 2 (paragraph 0048) discloses a technique in which a double-sided tape in which an adhesive layer is formed on both faces of a base material such as a PET film is bonded to a surface of the graphite sheet in advance and the graphite sheet is bonded to a target surface and part (hereinafter referred to as surface to be covered) of the electronic appliance through the adhesive layer. However, the adhesive layer is not usually thermal conductive, and tends to reduce a heat diffusion effect of the whole sheet. An acrylic adhesive layer usually used as the adhesive layer is difficult to be reworked (to be peeled once for re-adhering).
In Patent Literature 3, a graphite sheet having an elastic layer, such as silicone rubber (corresponding to the adhesive layer), containing a thermally conductive material is provided on a surface (claim 1) to try to prevent a decrease in thermal conductivity and solve reworking problem.
In an image panel, and in particular, an image display panel such as an OLED panel (organic EL panel), a surface constituting the panel is made of glass and is very smooth. However, when the adhesive layer contains the thermally conductive material, the surface of the adhesive layer is rough. Therefore, the initial adhesion may be reduced. Although this reduced initial adhesion contributes to reworking, the ratio of a resin component constituting the adhesive layer is relatively decreased. Therefore, a reduction in adhesion reliability is not avoided (problem 1).
When the graphite sheet has a thickness of 300 μm or less, this is not a serious problem. However, when the thickness of the graphite sheet is larger than that, the rigidity of the whole sheet increases, and as a result, it is difficult for the sheet to be bent during reworking.
Therefore, the graphite sheet is reworked (pulled upward) so that an angle from the target surface is not formed. However, in this case, a silicone adhesive layer tends to result in a cohesive failure, and as a result, tends to remain on the surface to be covered (problem 2).